ASK TOM WHY:

What causes the water level of Lake Michigan-Huron to fall?

You said Lake Michigan-Huron water levels are within 12 inches of their record low. It's obvious that precipitation causes the level to rise, but what causes it to fall?

—John Gleich, Highwood

Dear John,

Fluctuations in the two lakes' water levels are complex. Runoff (rivers) into the Michigan-Huron system, input from Lake Superior, precipitation, inflow of ground water and condensation directly onto the water surface contribute to rises in lake levels. Declines in the water level result from evaporation, drainage out of Huron via the St. Clair River and artificial diversions. Driven mainly by annual cycles of precipitation and evaporation (lighter precipitation and greater evaporation in winter; heavier precipitation and less evaporation in summer), Michigan-Huron levels annually fluctuate about one foot, falling from September through February and rising from March through August.